[Show abstract][Hide abstract] ABSTRACT: The performance degradation in convolutionally coded V-BLAST system is analyzed in this paper. Compared with coded linear filtering approach, it is shown that the coded V-BLAST suffers from serious degradation due to the effect of error propagation. Considering the constraint of the system, a performance improvement method is proposed. It adjusts the weighting of soft bits, where the weighting for one stream is found based on the equivalent error rate of that stream. Additional computation needed by the proposed method is neglectable. Both theoretical analysis and simulation verify the effectiveness of the proposed method.
Proceedings of the 67th IEEE Vehicular Technology Conference, VTC Spring 2008, 11-14 May 2008, Singapore; 05/2008
[Show abstract][Hide abstract] ABSTRACT: Mode selection problem in MIMO-OFDM system is addressed. A general formulation is introduced to decompose the problem into two decoupled sub-problems: modulation and coding scheme (MCS) selection, and space-time processing (STP) optimization, each of which can be tackled more systematically. Mode selection algorithms can then be developed using a channel quality indicator (CQI) metric. The methodology is illustrated using the platform of IEEE 802.11n. The qualities of multiple CQI metrics are compared in terms of "coherency" and their performances are evaluated in terms of system goodput. This has been shown that the metric we proposed outperforms the other metrics.
WCNC 2008, IEEE Wireless Communications & Networking Conference, March 31 2008 - April 3 2008, Las Vegas, Nevada, USA, Conference Proceedings; 01/2008
[Show abstract][Hide abstract] ABSTRACT: This paper considers the optimal resource allocation for a multiuser MIMO-OFDMA downlink system using zero-forcing SDMA, with the objective of maximizing the system capacity. We provide the general mathematical formulation of the capacity maximization problem whose resource allocation includes user selection, power allocation and receive signal subspace selection. While the optimal solution of this problem is difficult to obtain, we consider two special cases: user selection based only and antenna selection based only optimization, and develop the optimal solution by reformulating the combinatorial optimization problem into a convex problem. Our proposed algorithms for these special cases can be extended to develop a suboptimal algorithm for the original general optimization problem which allows subspace selection. Moreover, we develop reduced-complexity algorithms for these problems which incur negligible capacity loss. Numerical results and analysis are provided to evaluate the system capacity and to study the effect of the number of users and the receive antennas per user on the performance. Comparisons on the capacity achieved by user selection only, antenna selection only and suboptimal subspace selection are discussed
[Show abstract][Hide abstract] ABSTRACT: Orthogonal frequency division multiple access (OFDMA) and multicarrier code division multiple access (MC-CDMA) have recently drawn much attention for being potential candidates of future generation cellular systems. In single-cell scenario, while MC-CDMA is good at achieving frequency diversity when there is no channel state information available at the transmit side (CSIT), OFDMA achieves higher capacity than MC-CDMA due to its finer resolution in exploiting multiuser diversity with CSIT. Whether multiple-input-multiple-output (MIMO) MC-CDMA or OFDMA is a better option in multi-cell system remains unjustified in the literature. In this paper, we study the ergodic capacity and the goodput of MIMO-MC-CDMA and MIMO-OFDMA downlink systems with CSIT in multi-cell scenario assuming that the base station has the knowledge of the average inter-cell interference level only. Several types of users modeling different interference patterns are considered: (I) high data rate delay-insensitive users, (II) high data rate delay-sensitive .users, and (III) voice users (low data rate and bursty). Optimal resource allocation algorithms are used to compute the capacities of the systems, while a simple heuristic is used to obtain the achievable goodputs for both systems. The effects of path loss, number of antennas and different user types are studied and insightful results are obtained. We find that OFDMA has a higher system goodput for both Type I and Type II high data rate users, while MC-CDMA has a higher goodput for Type III users. Compared to MC-CDMA, the goodput of an OFDMA system is more sensitive to the activity factor of the voice users and suffers from noticeable loss. This demonstrates the superiority of the two systems in different practical situations.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we examine and compare the potential maximum sum capacity of downlink multiple-input-multiple- output orthogonal frequency division multiple access (MIMO- OFDMA) and multiple-input-multiple-output multicarrier code division multiple access (MIMO-MC-CDMA) in a single-cell multiuser environment with channel side information at the transmitter, with and without a fairness constraint. The resource allocation is formulated as a cross-layer optimization framework and optimal power allocation and user selection algorithms are proposed for both scenarios. We find that for delay-sensitive applications, where fairness is imposed, the performance gain of OFDMA over MC-CDMA is quite large at moderate path loss exponents and number of antennas. However, for delay- insensitive applications, the benefits of OFDMA over MC-CDMA are significantly reduced when the path loss exponent or the number of antennas is large.
[Show abstract][Hide abstract] ABSTRACT: Frequency-division duplexing and time-division duplexing are two common duplexing methods used in various wireless systems. However, there are advantages and technical issues associated with them. In this article we discuss in detail the features, and the design and implementation challenges of FDD and TDD systems for 4G wireless systems. In particular, we present a number of advantages and flexibilities an TDD system can bring to 4G systems that an FDD system cannot offer, and identify the major challenges, including cross-slot interference, in applying TDD in practice. Due to the fact that cross-slot interference is one of the critical challenges to employing TDD in cellular networks, we also provide a quantitative analysis on its impact on co-channel and adjacent channel interfering cells
[Show abstract][Hide abstract] ABSTRACT: This paper considers the optimal power allocation for a multiuser MIMO-OFDM downlink system using zero-forcing multiplexing, with the objective to maximize the system capacity. First, we consider the problem of user selection and power allocation for every subcarrier. By converting this combinatorial optimization problem into a convex one, an optimal power allocation algorithm has been derived. Then we introduce and formulate two new problems by extending user selection to antenna selection and dimension selection. We also provide optimal and suboptimal solutions to antenna selection and dimension selection problems respectively, by slightly modifying the proposed algorithm for user selection. Numerical results and analysis are provided to evaluate the system capacity and to study the effect of the number of users and the receive antennas per user on the performance. Comparison among user selection, antenna selection and dimension selection is made.